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Insulating front of oven landing

06-04-2011, 09:58 AM

Having not built an oven yet I have a question that I can't find an answer to in the FB plans. I will try to make my self clear. Everyone uses firebrick or some refractory material for their landing. Most people have a firebrick arch covering the vent landing that then transitions to whatever decorative arch is on the front of the enclosure. There is a layer of insulation beneath. Most people don't put any insulation between the fire brick of the vent landing and the concrete or tile or whatever other material is in front of it. Does this area get hot?

My oven has an hearth that extends 400mm (16ins) out from the dome entrance. I insulated mine because... I can remember exactly why now probably just your standard backyard overkill. But I'm glad I did as I oftern keep a fire in the entrance for everone to sit around

Comment

The landing will heat up from radiant heat regardless of the heat break.

My landing extends out considerably from the entrance. After a long session of baking pizzas the area directly in front of the entrance on my WFO gets to about 250 F. This is a measurement taken via infrared thermometer several inches out from the opening. At first I was concerned about stresses on the single piece of granite I use as my entrance, but so far so good, no cracking. The slab of granite measures 67 inches long 21 1/2 inches wide and 13/16 inches thick. The portion exposed to the interior at the entrance is 23 inches wide and centered.

Hope this helps,
Wiley

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I think that one of the advantages of having a heat break, if it is made of material that is somewhat elastic like vermicrete, is that it reduces expansion stresses on the outer parts of the oven. We're always banging on about insulating the oven, yet most designs connect the inner oven to the outer arch without any expansion joint or thermal break.

Kindled with zeal and fired with passion.

Comment

My vent landing area (casa80) is level with and contiguous with my concrete counter top.

Yes, it gets pretty hot from radiant heat - but not too bad. In a way I don't want to insulate this, as it is not really part of the retained heat system, even though it is conductively connected - I don't consider it strongly linked.

Comment

We're always banging on about insulating the oven, yet most designs connect the inner oven to the outer arch without any expansion joint or thermal break.

I have two thoughts about this. Regardless of whether or not a landing thermal break exists, the landing and archway will heat up from oven exhaust. This is not necessarily a bad thing.

The reason I designed a thermal break between my oven and archway, including landing, is twofold: stress-relief on oven dome expansion and reduction of residual heat loss through a connected archway.

Heating of the archway from oven exhaust is a reality. Theoretically, since heat transfer rate is partially a function of the difference in temperature of the two materials trading heat, the hotter the landing and archway, the slower the rate of heat transfer from the oven. This would make a case for employing a second outer door even though the vent and flue are still exposed to the elements.

In a way I don't want to insulate this, as it is not really part of the retained heat system, even though it is conductively connected - I don't consider it strongly linked.

I think SCChris studied this and discovered measurable oven heat loss through a contiguous firebrick landing. This loss was further supported by numbers suggesting a higher loss rate with soapstone (6x heat transfer-rate than firebrick). This analysis is what prompted me to design a heat-break in the landing as well as the oven and archway.

Comment

The short answer is that the landing under the vent does get hot. I can fry a pan of Italian sausage bits there while the fire is heating up, just from the radiant heat. I'm not convinced that a thermal break in the floor is advisable, particularly if there is exposed insulation. This area gets a LOT of traffic, with logs, and tools and pans and the tuscan grill sliding back and forth.

If you do a lot of retained heat cooking you might look into a thermal break. For pizza, I wouldn't bother.

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How about the outer arch? My original plan was to leave it exposed. We like the look of the firebrick arch. How hot can I expect it to get? Do I need to build another arch out front with a thermal break in between?

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I am not sure that I have been entirely clear. I understand that the firebrick on the vent landing is going to get hot enough to cook on. I was referring to the area in front of the external arch. Some folks like Dino and the person who did My Old Kentucky Dome have a counter in front of the oven that is usually concrete. I get the feeling that there is sufficient radiant heat from the oven to heat this area up regarless of whether or not there is a heat break. Does that make sense?
Bevan

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I put an eighth inch air space between the inner arch and the entry arch. You can see the daylight though the flu and the heat brake in this picture...

I cut rigid insulation and placed it on the outside radius of the air space, then covered it with vermicrete to seal and insulate. Haven't been over 600 F yet, but there is a large temperature difference between the inner arch and entry arch. For me, it was all to preserve heat for baking.

ADDED later: After the sixth day of curing fires my oven dome temp was 830 F, (433 C ), for an hour with a floor temp of 650 F, (343 C ). The top brick of the inner arch was 512 F, (266 C ), and the inner arch of the entry was 392 F, (200 C ). These numbers show the temperature difference between the inner arch and entry arch of 150 F, (66 C). Well worth the extra effort. In a do-over here, I might increase the air space to a quarter or half inch.

I made another air space in the oven floor about 4 inches outside my inner arch.
The air space will be between the firebricks and granite portion of the landing.
Also used a one inch filler between the oven floor and base for the granite.
The yard stick is where the granite goes.
This one shows the granite in place, but no entry yet. Designed after Karangi Dude's entry, I made mine a bit wider and a bit shallower than most. You can see the pencil line where I made the entry...

Added later: The temperature difference between the oven floor and granite was about 50 F, (10 C ), but I haven't filled that space with ashes yet, it may improve slightly. In any case it will aid the granite in a long and healthy life

Not shown is placement of two triangular insulated fire bricks that are now under bases of the entry, providing another degree of insulation.

Comment

Lee,
I don't think you'll be sorry you went to the trouble of doing what you've done. You will find that even after a few hours of firing that you can still hold your hand against the outer arch.You Will also appreciate the shallow entry every time you put anything in or out if the oven.

Kindled with zeal and fired with passion.

Comment

Lee,
I don't think you'll be sorry you went to the trouble of doing what you've done. You will find that even after a few hours of firing that you can still hold your hand against the outer arch.You Will also appreciate the shallow entry every time you put anything in or out if the oven.

Thanks for the encouragement David, I felt like I was in uncharted territory as I devised those measures....I did read all the heat brake threads I could find and gleaned from each published account. Actual implementation was not difficult at all, the biggest obstacle was to decide which materials to use and where -dictated by what I had on hand.

BTW, I also added some insulating firebrick between the top of the entry and the flu, and between the entry, the flu, and the block enclosure -got the idea from Brickie in Oz's latest build. I wanted to confine the heat and slow heat gain into the block enclosure.
Not very pretty I know, but I will be facing the entry and flu with granite, and the stand and enclosure with the white rock you see to the right of the entry, accented with brick from the house......Eventually.

Added later: During the fire reported above in post # 12, with the dome temp at 830 F, (433 C ), the clay flu approached 300 F, (148 C ). At this time the insulating fire brick at the base of the clay flu and between the flu and the block enclosure hovered around 160 F, (71 C ). This will ensure the desired effect of containing heat in the flu and slowing heat migration into the enclosure

P.S. The flu heats up well enough, despite the insulating bricks at its base, that was a concern -the second length of clay flu makes a LOT of difference in the draw and is necessary in our case.

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It is to me! Lee, your efforts to incorporate leading-edge elements into your oven shows forethought and a willingness to push the design evolution of the WFO. This is obviously a difficult process with mass-produced ovens.

I'm curious to learn how the double heat-break affects the outer arch and granite landing temps on a shallower-depth entryway and what kinds of heat-retention numbers you get. Keep the pics coming!